Grill the bratwurst over medium heat, about 3-5 minutes per side, or until the internal temperature is 160 degrees.
Meanwhile, in a large skillet, melt the butter and cook the onion until soft, about 5 minutes. Add the sauerkraut, vinegar, brown sugar, and mustard. Stir to combine and cook for 1-2 minutes until heated through.
Serve the brats on top of the kraut or with buns.
Canned sauerkraut does not need to be rinsed before using. Rinsing will remove some of the flavor, so only rinse if you prefer a milder taste.